With the limited spectrum space available for television communications links the occasion frequently arises when it would be desirable to transmit two television signals over a channel initially designed to carry only a single such signal. In the case of microwave or satellite links, or cable or other point-to-point links such a capability would double the transmission capacity of the channel without additional cost of transmission channel equipment as well as avoiding the need for additional frequency allocations in the case of radio links.
Where the signals are not intended for direct reception by the general public it is not necessary that the transmitted signal conform to broadcasting standards, provided that, if it is intended for subsequent broadcasting, it can be converted to broadcast standards after transmission over the link.
If the available channel bandwidth is split in two in order to convey two signals, each signal will have only half the total bandwidth. If this total bandwidth was initially intended for a single signal, each of the two television signals will suffer severe loss of resolution due to the halving of the bandwidth for each channel. Similar problems arise when attempts are made to time-share two signals on one channel on an element-by-element, line-by-line, or field-by-field basis. Resolution in the two signals, and/or the capability for portraying motion, is degraded.